1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent (EL) display device and a method of fabricating the same.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic electroluminescent (EL) devices have been of particular interest in research and development because they are light-emitting type displays having a wide viewing angle as well as a high contrast ratio in comparison to liquid crystal display (LCD) devices. Organic EL devices are lightweight and small, as compared to other types of display devices, because they do not need a backlight. Organic EL devices have other desirable characteristics, such as low power consumption, superior brightness and fast response time. When driving the organic EL devices, only a low direct current (DC) voltage is required. Moreover, a fast response time can be obtained. Unlike LCD devices, organic EL devices are entirely formed in a solid phase arrangement. Thus, organic EL devices are sufficiently strong to withstand external impacts and also have a greater operational temperature range. Moreover, organic EL devices are fabricated in a relatively simple process involving few processing steps. Thus, it is much cheaper to produce an organic EL device in comparison to an LCD device or a plasma display panel (PDP). In particular, only deposition and encapsulation processes are necessary for manufacturing organic EL devices.
There are two types of organic EL display devices: passive matrix type and active matrix type. While both the passive matrix organic EL display device and the active matrix organic EL display device have simple structures and are formed by a simple fabricating process, the passive matrix organic EL display device requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic EL display device is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic EL display device decreases. In contrast, active matrix organic EL display devices are highly efficient and can produce a high-quality image for a large display with a relatively low power.
FIG. 1 is a schematic circuit diagram of an organic electroluminescent display device according to the related art. In FIG. 1, a switching thin film transistor (TFT) “TS” as an addressing element is connected to a gate line 22 and a data line 40. A storage capacitor “CST” is connected to the switching TFT “TS,” and a driving TFT “TD” as a current source element is connected to the switching TFT “TS” and the storage capacitor “CST.” The driving TFT “TD” is also connected to a power line 26 and an organic electroluminescent (EL) diode “E.” The switching TFT “TS” adjusts a voltage of a terminal of the driving TFT “TD” and the storage capacitor “CST” stores charges for the voltage of the terminal of the driving TFT “TD.”
FIG. 2 is a schematic plane view showing an organic electroluminescent display device according to the related art. In FIG. 2, a gate line 22 is formed along a first direction on a substrate, and a data line 40 is formed along a second direction intersected with the gate line 22, thereby defining a pixel region “P.” A power line 26 also is formed along the second direction and spaced apart from the data line 40. A gate insulating layer (not shown) is interposed between the gate line 22 and the data line 40.
In addition, a switching thin film transistor (TFT) “TS” is connected to the gate line 22 and the data line 40. A driving TFT “TD” is connected to the switching TFT “TS” and the power line 26, and a storage capacitor “CST” is connected to the driving TFT “TD.” A first electrode 46 is connected to driving TFT “TD,” and an organic luminescent layer (not shown) and a second electrode (not shown) are sequentially formed on the first electrode 46.
FIGS. 3A to 3I are schematic cross-sectional views, which is taken along a line III-III of FIG. 2, showing a fabricating process of an organic electroluminescent display device according to the related art.
In FIG. 3A, a buffer layer 12 is formed on a substrate 10. A semiconductor layer 14 of polycrystalline silicon and a first capacitor electrode 16 are formed on the buffer layer 12 through a first mask process.
In FIG. 3B, a first photoresist (PR) pattern 15 is formed on the semiconductor pattern 14 through a second mask process. Next, the first capacitor electrode 16 is doped with impurities using the first PR pattern as a doping mask.
In FIG. 3C, a gate insulating layer 18 is formed on the semiconductor pattern 14 and the first capacitor electrode 16. Next, a gate electrode 20 and a second capacitor electrode 24 are formed on the gate insulating layer 18 through a third mask process. The gate electrode 20 and the second capacitor electrode 24 correspond to a central portion of the semiconductor pattern 14 and the first capacitor electrode 16, respectively. Next, the semiconductor layer 14 is doped with impurities using the gate electrode 20 as a doping mask. Since the gate electrode 14 shields the central portion of the semiconductor layer 14, side portions of the semiconductor layer 14 are doped with impurities and the central portion of the semiconductor layer 14 remains intrinsic. After the doping step, the semiconductor layer 14 includes an active region “Sa” corresponding to the gate electrode 20, a source region “Sb” and a drain region “Sc.”
In FIG. 3D, after an activation step for the doped impurities, an interlayer insulating layer 28 is formed on the gate electrode 20 and the second capacitor electrode 24. Next, the interlayer insulating layer 28 and the gate insulating layer 18 are etched through a fourth mask process. Accordingly, a first contact hole 30 exposing the drain region “Sc” and a second contact hole 32 exposing the source region “Sb” are formed in the interlayer insulating layer 28 and the gate insulating layer 18, and a third contact hole 34 exposing the second capacitor electrode 24 is formed in the interlayer insulating layer 28.
In FIG. 3E, a drain electrode 36 and a source electrode 38 are formed on the interlayer insulating layer 28. The drain electrode 36 is connected to the drain region “Sc” through the first contact hole 30. In addition, the source electrode 38 is connected to the source region “Sb” through the second contact hole 32 and is connected to the second capacitor electrode 24 through the third contact hole 34.
In FIG. 3F, a passivation layer 42 is formed on the drain electrode 36 and the source electrode 38. Next, the passivation layer 42 is etched through a sixth mask process to form a drain contact hole 44 exposing the drain electrode 36.
In FIG. 3G, a first electrode 46 is formed on the passivation layer 42 through a seventh mask process. The first electrode 46 is connected to the drain electrode 36 through the drain contact hole 44. Although not shown in FIG. 3G, the first electrode 46 of one pixel region is separated from an adjacent first electrode in another pixel region.
In FIG. 3H, a bank layer 48 is formed on the passivation layer 42 through an eighth mask process. The bank layer 48 covers edge portions of the first electrode 46.
In FIG. 3I, an organic luminescent layer 50 and a second electrode 52 are sequentially formed on the first electrode 46 and the bank layer 48.
FIG. 4 is a schematic cross-sectional view of an organic electroluminescent display device according to the related art. In FIG. 4, an organic electroluminescent (EL) diode “E” is formed on an array layer “AL.” The organic EL diode “E” includes a first electrode 46, an organic luminescent layer 50 and a second electrode 52, and the array layer “AL” includes a thin film transistor (TFT) “T,” a storage capacitor “CST” and a plurality of signal lines (not shown). A protection layer 54 is formed on the organic EL diode “E” to protect the second electrode 52 of the organic EL diode “E.”
The TFT “T,” the storage capacitor “CST” and a plurality of signal lines (not shown) of the array layer “AL” include metallic materials. In addition, when the second electrode 52 functions as a cathode, a metallic material such as nickel (Ni) having a relatively low work function is used for the second electrode 52. Accordingly, ambient light may reflect from layer including the metallic materials during the organic EL display device is driven, and reflected light deteriorates display quality of the organic EL display device such as a contrast ratio. For example, when the organic EL display device has a top emission type, ambient light reflects from the second electrode 52 and a first reflected light “R1” is emitted toward users. Similarly, the ambient light reflects from the array layer “AL” such as the TFT “T” and the storage capacitor “CST,” and second and third reflected light “R2” and “R3” are emitted toward users. The first, second and third reflected light “R1,” “R2” and “R3” may increase brightness of a black image, thereby deteriorating a contrast ratio of the organic EL display device.
As referring again to FIG. 3I, a cleaning step using ultra violet (UV) ray is performed to remove organic particles from the first electrode 46 before the organic luminescent layer 50 on the first electrode 46. However, the UV ray may penetrate the bank layer 48, the passivation layer 42 and the interlayer insulating layer 28 to be irradiated onto the semiconductor layer 14. Accordingly, the semiconductor layer 14 may be deteriorated by the UV ray and the deterioration of the semiconductor layer 14 may cause degradation in characteristics of the TFT. For example, a threshold voltage of the TFT may be changed and a display quality of the organic EL display device is deteriorated.